To spoil and love your grandchildren.
To always be there when they need to talk, cry or be held.
To kiss the boo boo's.
To fight off the scary monsters.
To be proud their first day of school.
To be wow-ed their graduation day.
To reassure when they go to college.
But......custody / visitation / support.........In most cases, nothing. (Unless something is really wrong in the house) Be really, really nice to mom and dad. They decide what part you play in the lives of your grandbabies.